The goal of the proposed research is to determine how proteins at the interface of homologous recombinational repair (HR) and nonhomologous end-joining (NHEJ) modulate DNA double-strand break (DSB) repair fidelity. Defects in HR and NHEJ proteins are linked to cancer predisposition. DSBs are key DNA lesions produced by genotoxic chemicals and radiation, and DSBs may arise spontaneously during DNA replication. HR and NHEJ compete for the repair of DSBs, and the genetic consequences of the two repair pathways are significantly different. Many factors are likely to influence the choice between HR and NHEJ, some of which are substrate-dependent. Perhaps more important from the standpoint of potential targets for chemo- or radiotherapeutic intervention in cancer treatment are trans factors, i.e., DNA repair proteins, including their concentrations, physical interactions, and biochemical activities. HR and NHEJ may compete passively for DSBs, with each operating independently of the other. Alternatively, competition may be active, with HR proteins interacting with, and modulating the activities of NHEJ proteins, and vice versa. Although most DSB repair proteins have been assigned to the HR or the NHEJ pathway, some influence both pathways, such as the MRE11/RAD50/NBS1 (MRN) complex. Recent evidence indicates that DNAPKcs and Ku are also at the NHEJ/HR interface. These proteins therefore play important roles in determining the genetic consequences of DSB damage. Other data indicate that DNA-PKcs, Ku, and MRN also regulate spontaneous HR, suggesting another mechanism by which these proteins regulate genome stability. Our central hypothesis is that proteins at the HR/NHEJ interface regulate genome stability by controlling the relative levels and outcomes of NHEJ and HR during DSB repair, and by controlling spontaneous FIR levels. We will determine how DSB repair is regulated by DNA-PKcs (Aim 1), Ku (Aim 2), MRN (Aim 3), and we will determine if spontaneous HR is regulated by these proteins (Aim 4). These projects will provide insight into the regulation of mammalian DSB repair and spontaneous HR that can be exploited to develop more effective agents to sensitize tumor cells to DNA damaging agents, and thereby improve cancer therapy.